diff --git a/eslint.config.mjs b/eslint.config.mjs
index c634629..2dac3b5 100644
--- a/eslint.config.mjs
+++ b/eslint.config.mjs
@@ -17,6 +17,8 @@ const DYNAMIC_LOGGER_CALL_SELECTOR =
 const LOGGER_RECEIVER_SELECTOR =
   ":matches([callee.object.name=/^(logger|serverLogger)$/],[callee.object.property.name='logger'])";
 const SERVER_LOGGER_CALL_SELECTOR = `:matches(${STATIC_LOGGER_CALL_SELECTOR}${LOGGER_RECEIVER_SELECTOR},${DYNAMIC_LOGGER_CALL_SELECTOR}${LOGGER_RECEIVER_SELECTOR})`;
+const NEXT_RESPONSE_ERROR_JSON_SELECTOR =
+  "CallExpression[callee.type='MemberExpression'][callee.object.name='NextResponse'][callee.property.name='json'][arguments.0.type='ObjectExpression']:has(Property[key.name='error'])";
 
 const eslintConfig = [
   ...compat.extends("next/core-web-vitals", "next/typescript"),
@@ -87,6 +89,11 @@ const eslintConfig = [
           message:
             "Do not use top-level `detail` in server logger context; keep throwable text under `error.message`.",
         },
+        {
+          selector: `${SERVER_LOGGER_CALL_SELECTOR} > ObjectExpression:first-child > Property[key.name='errorCode']`,
+          message:
+            "Do not use top-level `errorCode` in server logger context; classify failures under nested `error.code`.",
+        },
         {
           selector: `${SERVER_LOGGER_CALL_SELECTOR} > ObjectExpression:first-child > Property[key.name='err']`,
           message:
@@ -102,6 +109,11 @@ const eslintConfig = [
           message:
             "Server logger `error` must be a structured object (prefer `errorToLog(...)`), not template text.",
         },
+        {
+          selector: `CatchClause ReturnStatement > ${NEXT_RESPONSE_ERROR_JSON_SELECTOR}[arguments.1.type='ObjectExpression']:has(Property[key.name='status'][value.value=500])`,
+          message:
+            "Use shared error response helpers (e.g. errorResponse(...)) for terminal 500 route failures instead of direct NextResponse.json({ error }).",
+        },
       ],
     },
   },
diff --git a/package.json b/package.json
index 74bcfa2..f509a63 100644
--- a/package.json
+++ b/package.json
@@ -25,7 +25,8 @@
     "compute:worker:dev": "pnpm --filter @openreader/compute-worker dev",
     "compute:worker:start": "pnpm --filter @openreader/compute-worker start",
     "compute:dev:compose": "docker compose --env-file compute/worker/.env -f compute/worker/docker-compose.yml up --build",
-    "compute:dev:watch": "docker compose --env-file compute/worker/.env -f compute/worker/docker-compose.yml up --watch"
+    "compute:dev:watch": "docker compose --env-file compute/worker/.env -f compute/worker/docker-compose.yml up --watch",
+    "lint:route-errors": "node scripts/check-route-error-responses.mjs"
   },
   "dependencies": {
     "@aws-sdk/client-s3": "^3.1045.0",
diff --git a/scripts/check-route-error-responses.mjs b/scripts/check-route-error-responses.mjs
new file mode 100644
index 0000000..8770958
--- /dev/null
+++ b/scripts/check-route-error-responses.mjs
@@ -0,0 +1,54 @@
+#!/usr/bin/env node
+import fs from 'node:fs/promises';
+import path from 'node:path';
+
+const ROOT = process.cwd();
+const API_DIR = path.join(ROOT, 'src', 'app', 'api');
+
+async function walk(dir) {
+  const entries = await fs.readdir(dir, { withFileTypes: true });
+  const files = [];
+  for (const entry of entries) {
+    const full = path.join(dir, entry.name);
+    if (entry.isDirectory()) {
+      files.push(...await walk(full));
+      continue;
+    }
+    if (entry.isFile() && full.endsWith('.ts')) files.push(full);
+  }
+  return files;
+}
+
+function findViolations(source) {
+  const violations = [];
+  const catchWithDirectJson500 =
+    /catch\s*\(\s*error\s*\)\s*\{[\s\S]{0,1200}?NextResponse\.json\s*\(\s*\{\s*error:[\s\S]{0,240}status:\s*500/g;
+  let match;
+  while ((match = catchWithDirectJson500.exec(source)) !== null) {
+    const index = match.index;
+    const line = source.slice(0, index).split('\n').length;
+    violations.push(line);
+  }
+  return violations;
+}
+
+const files = await walk(API_DIR);
+const failures = [];
+for (const file of files) {
+  const source = await fs.readFile(file, 'utf8');
+  const lines = findViolations(source);
+  if (lines.length > 0) {
+    failures.push({ file, lines });
+  }
+}
+
+if (failures.length > 0) {
+  for (const failure of failures) {
+    for (const line of failure.lines) {
+      console.error(`${path.relative(ROOT, failure.file)}:${line} direct NextResponse.json({ error }) in catch`);
+    }
+  }
+  process.exit(1);
+}
+
+console.log('Route error response check passed.');
diff --git a/src/app/api/account/delete/route.ts b/src/app/api/account/delete/route.ts
index c54505b..740aae5 100644
--- a/src/app/api/account/delete/route.ts
+++ b/src/app/api/account/delete/route.ts
@@ -5,6 +5,7 @@ import { isAuthEnabled } from '@/lib/server/auth/config';
 import { getOpenReaderTestNamespace } from '@/lib/server/testing/test-namespace';
 import { deleteUserStorageData } from '@/lib/server/user/data-cleanup';
 import { errorToLog, hashForLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 
 export async function DELETE() {
   if (!isAuthEnabled() || !auth) {
@@ -51,9 +52,9 @@ export async function DELETE() {
       event: 'account.delete.failed',
       error: errorToLog(error),
     }, 'Failed to delete account');
-    return NextResponse.json(
-      { error: 'Failed to delete account' },
-      { status: 500 }
-    );
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to delete account',
+      normalize: { code: 'ACCOUNT_DELETE_FAILED', errorClass: 'db' },
+    });
   }
 }
diff --git a/src/app/api/admin/providers/[id]/route.ts b/src/app/api/admin/providers/[id]/route.ts
index a3be4fb..c3755f6 100644
--- a/src/app/api/admin/providers/[id]/route.ts
+++ b/src/app/api/admin/providers/[id]/route.ts
@@ -1,6 +1,7 @@
 import { NextRequest, NextResponse } from 'next/server';
 import { requireAdminContext } from '@/lib/server/auth/admin';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 import {
   AdminProviderError,
   deleteAdminProvider,
@@ -47,7 +48,10 @@ export async function PUT(
       providerId: id,
       error: errorToLog(error),
     }, 'Admin provider update failed');
-    return NextResponse.json({ error: 'Internal error' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Internal error',
+      normalize: { code: 'ADMIN_PROVIDERS_UPDATE_FAILED', errorClass: 'db' },
+    });
   }
 }
 
@@ -71,6 +75,9 @@ export async function DELETE(
       providerId: id,
       error: errorToLog(error),
     }, 'Admin provider delete failed');
-    return NextResponse.json({ error: 'Internal error' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Internal error',
+      normalize: { code: 'ADMIN_PROVIDERS_DELETE_FAILED', errorClass: 'db' },
+    });
   }
 }
diff --git a/src/app/api/admin/providers/route.ts b/src/app/api/admin/providers/route.ts
index 778a6f7..96d8fa9 100644
--- a/src/app/api/admin/providers/route.ts
+++ b/src/app/api/admin/providers/route.ts
@@ -1,6 +1,7 @@
 import { NextRequest, NextResponse } from 'next/server';
 import { requireAdminContext } from '@/lib/server/auth/admin';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 import {
   AdminProviderError,
   createAdminProvider,
@@ -51,6 +52,9 @@ export async function POST(req: NextRequest) {
       event: 'admin.providers.create.failed',
       error: errorToLog(error),
     }, 'Admin provider create failed');
-    return NextResponse.json({ error: 'Internal error' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Internal error',
+      normalize: { code: 'ADMIN_PROVIDERS_CREATE_FAILED', errorClass: 'db' },
+    });
   }
 }
diff --git a/src/app/api/audiobook/chapter/route.ts b/src/app/api/audiobook/chapter/route.ts
index 20b48e5..c2a2f27 100644
--- a/src/app/api/audiobook/chapter/route.ts
+++ b/src/app/api/audiobook/chapter/route.ts
@@ -44,6 +44,7 @@ import {
   coerceAudiobookGenerationSettings,
   type SharedProviderPolicyEntry,
 } from '@/lib/server/audiobooks/settings';
+import { errorResponse } from '@/lib/server/errors/next-response';
 
 export const dynamic = 'force-dynamic';
 
@@ -340,7 +341,10 @@ export async function POST(request: NextRequest) {
             message: 'Invalid audiobook.meta.json settings payload',
           },
         }, 'Invalid audiobook.meta.json settings payload');
-        return NextResponse.json({ error: 'Invalid audiobook metadata settings' }, { status: 500 });
+        return errorResponse(new Error('Invalid audiobook metadata settings payload'), {
+          apiErrorMessage: 'Invalid audiobook metadata settings',
+          normalize: { code: 'AUDIOBOOK_CHAPTER_META_SETTINGS_INVALID', errorClass: 'validation', httpStatus: 500 },
+        });
       }
       normalizedExistingSettings = normalizeNativeSpeedForSettings(existingResult.settings);
       existingSettingsNeedsMigration = existingResult.migrated;
@@ -419,7 +423,7 @@ export async function POST(request: NextRequest) {
           step: 'persist_migrated_settings',
           bookId,
           storageUserId,
-          error: error instanceof Error ? error.message : String(error),
+          error: errorToLog(error),
         }, 'Failed to persist migrated audiobook metadata settings');
       }
     }
@@ -503,7 +507,10 @@ export async function POST(request: NextRequest) {
     }
     const provider = credResolved.provider;
     if (!isBuiltInTtsProviderId(provider)) {
-      return NextResponse.json({ error: `Unsupported TTS provider type: ${provider}` }, { status: 500 });
+      return errorResponse(new Error(`Unsupported TTS provider type: ${provider}`), {
+        apiErrorMessage: `Unsupported TTS provider type: ${provider}`,
+        normalize: { code: 'AUDIOBOOK_CHAPTER_UNSUPPORTED_PROVIDER', errorClass: 'validation', httpStatus: 500 },
+      });
     }
     const openApiKey = credResolved.apiKey || 'none';
     const openApiBaseUrl = credResolved.baseUrl;
@@ -754,7 +761,10 @@ export async function POST(request: NextRequest) {
       event: 'audiobook.chapter.process.failed',
       error: errorToLog(error),
     }, 'Failed to process audio chapter');
-    const response = NextResponse.json({ error: 'Failed to process audio chapter' }, { status: 500 });
+    const response = errorResponse(error, {
+      apiErrorMessage: 'Failed to process audio chapter',
+      normalize: { code: 'AUDIOBOOK_CHAPTER_PROCESS_FAILED', errorClass: 'upstream' },
+    });
     attachDeviceIdCookie(response, deviceIdToSet, didCreateDeviceIdCookie);
     return response;
   } finally {
@@ -852,7 +862,10 @@ export async function GET(request: NextRequest) {
       event: 'audiobook.chapter.download.failed',
       error: errorToLog(error),
     }, 'Failed to download chapter');
-    return NextResponse.json({ error: 'Failed to download chapter' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to download chapter',
+      normalize: { code: 'AUDIOBOOK_CHAPTER_DOWNLOAD_FAILED', errorClass: 'storage' },
+    });
   }
 }
 
@@ -923,6 +936,9 @@ export async function DELETE(request: NextRequest) {
       event: 'audiobook.chapter.delete.failed',
       error: errorToLog(error),
     }, 'Failed to delete chapter');
-    return NextResponse.json({ error: 'Failed to delete chapter' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to delete chapter',
+      normalize: { code: 'AUDIOBOOK_CHAPTER_DELETE_FAILED', errorClass: 'db' },
+    });
   }
 }
diff --git a/src/app/api/audiobook/route.ts b/src/app/api/audiobook/route.ts
index d8e5aea..0b2a5da 100644
--- a/src/app/api/audiobook/route.ts
+++ b/src/app/api/audiobook/route.ts
@@ -8,6 +8,7 @@ import { db } from '@/db';
 import { audiobooks, audiobookChapters } from '@/db/schema';
 import { requireAuthContext } from '@/lib/server/auth/auth';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 import {
   audiobookPrefix,
   deleteAudiobookObject,
@@ -380,7 +381,10 @@ export async function GET(request: NextRequest) {
       event: 'audiobook.create.failed',
       error: errorToLog(error),
     }, 'Failed to create full audiobook');
-    return NextResponse.json({ error: 'Failed to create full audiobook file' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to create full audiobook file',
+      normalize: { code: 'AUDIOBOOK_CREATE_FAILED', errorClass: 'upstream' },
+    });
   } finally {
     if (workDir) await rm(workDir, { recursive: true, force: true }).catch(() => {});
   }
@@ -431,6 +435,9 @@ export async function DELETE(request: NextRequest) {
       event: 'audiobook.reset.failed',
       error: errorToLog(error),
     }, 'Failed to reset audiobook');
-    return NextResponse.json({ error: 'Failed to reset audiobook' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to reset audiobook',
+      normalize: { code: 'AUDIOBOOK_RESET_FAILED', errorClass: 'db' },
+    });
   }
 }
diff --git a/src/app/api/audiobook/status/route.ts b/src/app/api/audiobook/status/route.ts
index 49e30dc..29bda02 100644
--- a/src/app/api/audiobook/status/route.ts
+++ b/src/app/api/audiobook/status/route.ts
@@ -12,6 +12,7 @@ import { buildAllowedAudiobookUserIds, pickAudiobookOwner } from '@/lib/server/a
 import type { AudiobookGenerationSettings } from '@/types/client';
 import type { TTSAudiobookChapter, TTSAudiobookFormat } from '@/types/tts';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 
 export const dynamic = 'force-dynamic';
 
@@ -160,6 +161,9 @@ export async function GET(request: NextRequest) {
       event: 'audiobook.status.fetch.failed',
       error: errorToLog(error),
     }, 'Failed to fetch audiobook chapters');
-    return NextResponse.json({ error: 'Failed to fetch chapters' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to fetch chapters',
+      normalize: { code: 'AUDIOBOOK_STATUS_FETCH_FAILED', errorClass: 'db' },
+    });
   }
 }
diff --git a/src/app/api/documents/[id]/parsed/events/route.ts b/src/app/api/documents/[id]/parsed/events/route.ts
index 7d2109c..eb7e550 100644
--- a/src/app/api/documents/[id]/parsed/events/route.ts
+++ b/src/app/api/documents/[id]/parsed/events/route.ts
@@ -11,7 +11,9 @@ import { normalizeParseStatus, parseDocumentParseState } from '@/lib/server/docu
 import { healStaleDocumentParseState } from '@/lib/server/documents/parse-state-healing';
 import { getOpenReaderTestNamespace, getUnclaimedUserIdForNamespace } from '@/lib/server/testing/test-namespace';
 import { isS3Configured } from '@/lib/server/storage/s3';
-import { createRequestLogger, errorToLog, hashForLog } from '@/lib/server/logger';
+import { createRequestLogger, hashForLog } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
+import { logDegraded, logServerError } from '@/lib/server/errors/logging';
 import type { PdfParseProgress, PdfParseStatus } from '@/types/parsed-pdf';
 import { parseSseEventId, parseSsePayload } from '@openreader/compute-core';
 import type { PdfLayoutJobResult, WorkerOperationEvent, WorkerOperationState } from '@openreader/compute-core/api-contracts';
@@ -267,15 +269,17 @@ export async function GET(req: NextRequest, ctx: { params: Promise<{ id: string
               }
             }).catch((error) => {
               if (closed) return;
-              logger.warn({
+              logDegraded(logger, {
                 event: 'documents.parsed.events.db_resync_failed',
-                degraded: true,
+                msg: 'SSE DB resync failed',
                 step: 'db_resync',
-                documentId: id,
-                storageUserIdHash,
-                requestId,
-                error: errorToLog(error),
-              }, 'SSE DB resync failed');
+                context: {
+                  documentId: id,
+                  storageUserIdHash,
+                  requestId,
+                },
+                error,
+              });
               controller.enqueue(encoder.encode(`event: error\ndata: ${JSON.stringify({ error: String(error) })}\n\n`));
             });
           }, SSE_RESYNC_INTERVAL_MS);
@@ -468,11 +472,16 @@ export async function GET(req: NextRequest, ctx: { params: Promise<{ id: string
 
         void runWorkerProxy()
           .catch((error) => {
-            logger.error({
+            logServerError(logger, {
               event: 'documents.parsed.events.worker_proxy_crashed',
-              documentId: id,
-              error: errorToLog(error),
-            }, 'Worker proxy crashed while streaming parse events');
+              error,
+              msg: 'Worker proxy crashed while streaming parse events',
+              context: { documentId: id },
+              normalize: {
+                code: 'DOCUMENTS_PARSED_EVENTS_WORKER_PROXY_CRASHED',
+                errorClass: 'upstream',
+              },
+            });
             if (!closed) {
               controller.enqueue(encoder.encode(`event: error\ndata: ${JSON.stringify({ error: String(error) })}\n\n`));
             }
@@ -499,10 +508,12 @@ export async function GET(req: NextRequest, ctx: { params: Promise<{ id: string
       },
     });
   } catch (error) {
-    logger.error({
+    return errorResponse(error, {
+      logger,
       event: 'documents.parsed.events.route_failed',
-      error: errorToLog(error),
-    }, 'Parsed events route failed');
-    return NextResponse.json({ error: 'Failed to stream parsed PDF progress' }, { status: 500 });
+      msg: 'Parsed events route failed',
+      apiErrorMessage: 'Failed to stream parsed PDF progress',
+      normalize: { code: 'DOCUMENTS_PARSED_EVENTS_ROUTE_FAILED', errorClass: 'upstream' },
+    });
   }
 }
diff --git a/src/app/api/documents/[id]/parsed/route.ts b/src/app/api/documents/[id]/parsed/route.ts
index d3e8539..51ef630 100644
--- a/src/app/api/documents/[id]/parsed/route.ts
+++ b/src/app/api/documents/[id]/parsed/route.ts
@@ -23,7 +23,9 @@ import {
 import { healStaleDocumentParseState } from '@/lib/server/documents/parse-state-healing';
 import { getOpenReaderTestNamespace, getUnclaimedUserIdForNamespace } from '@/lib/server/testing/test-namespace';
 import { isS3Configured } from '@/lib/server/storage/s3';
-import { createRequestLogger, errorToLog, hashForLog, type ServerLogger } from '@/lib/server/logger';
+import { createRequestLogger, hashForLog, type ServerLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
+import { logDegraded } from '@/lib/server/errors/logging';
 import type { ParsedPdfDocument } from '@/types/parsed-pdf';
 import type { PdfLayoutJobResult, WorkerOperationState } from '@openreader/compute-core/api-contracts';
 
@@ -138,7 +140,10 @@ async function finalizeFromWorkerState(input: {
   }
 
   if (!parsedJsonKey) {
-    return NextResponse.json({ error: 'Worker completed without parsed output' }, { status: 500 });
+    return errorResponse(new Error('Worker completed without parsed output'), {
+      apiErrorMessage: 'Worker completed without parsed output',
+      normalize: { code: 'DOCUMENTS_PARSED_WORKER_OUTPUT_MISSING', errorClass: 'upstream' },
+    });
   }
 
   await writeParseRowState({
@@ -218,18 +223,20 @@ export async function GET(req: NextRequest, ctx: { params: Promise<{ id: string
           logger,
         });
       }
-      logger.warn({
+      logDegraded(logger, {
         event: 'documents.parsed.requested_op_unavailable',
-        degraded: true,
+        msg: 'Requested worker operation id was unavailable',
         step: 'requested_op_lookup',
-        documentId: id,
-        userIdHash: hashForLog(row.userId),
-        opId: requestedOpId,
-        error: {
-          name: 'RequestedOperationUnavailable',
-          message: `Requested worker operation ${requestedOpId} was unavailable`,
+        context: {
+          documentId: id,
+          userIdHash: hashForLog(row.userId),
+          opId: requestedOpId,
+          error: {
+            name: 'RequestedOperationUnavailable',
+            message: `Requested worker operation ${requestedOpId} was unavailable`,
+          },
         },
-      }, 'Requested worker operation id was unavailable');
+      });
     }
 
     let state = parseDocumentParseState(row.parseState);
@@ -311,11 +318,13 @@ export async function GET(req: NextRequest, ctx: { params: Promise<{ id: string
       throw error;
     }
   } catch (error) {
-    logger.error({
+    return errorResponse(error, {
+      logger,
       event: 'documents.parsed.get_failed',
-      error: errorToLog(error),
-    }, 'Failed to read parsed PDF');
-    return NextResponse.json({ error: 'Failed to read parsed PDF' }, { status: 500 });
+      msg: 'Failed to read parsed PDF',
+      apiErrorMessage: 'Failed to read parsed PDF',
+      normalize: { code: 'DOCUMENTS_PARSED_GET_FAILED', errorClass: 'storage' },
+    });
   }
 }
 
@@ -391,10 +400,12 @@ export async function POST(req: NextRequest, ctx: { params: Promise<{ id: string
       opId: created.opId,
     }, { status: 202 });
   } catch (error) {
-    logger.error({
+    return errorResponse(error, {
+      logger,
       event: 'documents.parsed.force_refresh_failed',
-      error: errorToLog(error),
-    }, 'Failed to force PDF refresh');
-    return NextResponse.json({ error: 'Failed to force PDF refresh' }, { status: 500 });
+      msg: 'Failed to force PDF refresh',
+      apiErrorMessage: 'Failed to force PDF refresh',
+      normalize: { code: 'DOCUMENTS_PARSED_FORCE_REFRESH_FAILED', errorClass: 'upstream' },
+    });
   }
 }
diff --git a/src/app/api/documents/[id]/settings/route.ts b/src/app/api/documents/[id]/settings/route.ts
index 594da97..5b94e23 100644
--- a/src/app/api/documents/[id]/settings/route.ts
+++ b/src/app/api/documents/[id]/settings/route.ts
@@ -8,6 +8,7 @@ import { mergeDocumentSettings } from '@/lib/shared/document-settings';
 import { DEFAULT_DOCUMENT_SETTINGS, type DocumentSettings } from '@/types/document-settings';
 import { coerceTimestampMs, nowTimestampMs } from '@/lib/shared/timestamps';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 
 export const dynamic = 'force-dynamic';
 
@@ -97,7 +98,10 @@ export async function GET(req: NextRequest, ctx: { params: Promise<{ id: string
       event: 'documents.settings.load.failed',
       error: errorToLog(error),
     }, 'Failed to load document settings');
-    return NextResponse.json({ error: 'Failed to load document settings' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to load document settings',
+      normalize: { code: 'DOCUMENTS_SETTINGS_LOAD_FAILED', errorClass: 'db' },
+    });
   }
 }
 
@@ -169,6 +173,9 @@ export async function PUT(req: NextRequest, ctx: { params: Promise<{ id: string
       event: 'documents.settings.update.failed',
       error: errorToLog(error),
     }, 'Failed to update document settings');
-    return NextResponse.json({ error: 'Failed to update document settings' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to update document settings',
+      normalize: { code: 'DOCUMENTS_SETTINGS_UPDATE_FAILED', errorClass: 'db' },
+    });
   }
 }
diff --git a/src/app/api/documents/blob/get/fallback/route.ts b/src/app/api/documents/blob/get/fallback/route.ts
index 75e6f18..5584cfc 100644
--- a/src/app/api/documents/blob/get/fallback/route.ts
+++ b/src/app/api/documents/blob/get/fallback/route.ts
@@ -8,6 +8,7 @@ import { getDocumentBlob, isMissingBlobError, isValidDocumentId } from '@/lib/se
 import { getOpenReaderTestNamespace, getUnclaimedUserIdForNamespace } from '@/lib/server/testing/test-namespace';
 import { isS3Configured } from '@/lib/server/storage/s3';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 
 export const dynamic = 'force-dynamic';
 
@@ -91,6 +92,9 @@ export async function GET(req: NextRequest) {
       event: 'documents.blob.get.fallback.failed',
       error: errorToLog(error),
     }, 'Failed to load document content fallback');
-    return NextResponse.json({ error: 'Failed to load document content' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to load document content',
+      normalize: { code: 'DOCUMENTS_BLOB_GET_FALLBACK_FAILED', errorClass: 'storage' },
+    });
   }
 }
diff --git a/src/app/api/documents/blob/get/presign/route.ts b/src/app/api/documents/blob/get/presign/route.ts
index b6c30e8..8346c3c 100644
--- a/src/app/api/documents/blob/get/presign/route.ts
+++ b/src/app/api/documents/blob/get/presign/route.ts
@@ -7,6 +7,7 @@ import { isValidDocumentId, presignGet } from '@/lib/server/documents/blobstore'
 import { getOpenReaderTestNamespace, getUnclaimedUserIdForNamespace } from '@/lib/server/testing/test-namespace';
 import { isS3Configured } from '@/lib/server/storage/s3';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 
 export const dynamic = 'force-dynamic';
 
@@ -72,6 +73,9 @@ export async function GET(req: NextRequest) {
       event: 'documents.blob.get.presign.failed',
       error: errorToLog(error),
     }, 'Failed to create document download signature');
-    return NextResponse.json({ error: 'Failed to prepare document download' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to prepare document download',
+      normalize: { code: 'DOCUMENTS_BLOB_GET_PRESIGN_FAILED', errorClass: 'storage' },
+    });
   }
 }
diff --git a/src/app/api/documents/blob/preview/ensure/route.ts b/src/app/api/documents/blob/preview/ensure/route.ts
index 802a868..1fa3450 100644
--- a/src/app/api/documents/blob/preview/ensure/route.ts
+++ b/src/app/api/documents/blob/preview/ensure/route.ts
@@ -3,6 +3,7 @@ import { presignDocumentPreviewGet } from '@/lib/server/documents/previews-blobs
 import { ensureDocumentPreview } from '@/lib/server/documents/previews';
 import { validatePreviewRequest } from '../utils';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 
 export const dynamic = 'force-dynamic';
 
@@ -55,6 +56,9 @@ export async function GET(req: NextRequest) {
       event: 'documents.preview.ensure.failed',
       error: errorToLog(error),
     }, 'Failed to ensure document preview');
-    return NextResponse.json({ error: 'Failed to ensure document preview' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to ensure document preview',
+      normalize: { code: 'DOCUMENTS_PREVIEW_ENSURE_FAILED', errorClass: 'storage' },
+    });
   }
 }
diff --git a/src/app/api/documents/blob/preview/fallback/route.ts b/src/app/api/documents/blob/preview/fallback/route.ts
index 09f0851..a83955e 100644
--- a/src/app/api/documents/blob/preview/fallback/route.ts
+++ b/src/app/api/documents/blob/preview/fallback/route.ts
@@ -4,6 +4,7 @@ import { db } from '@/db';
 import { documents } from '@/db/schema';
 import { requireAuthContext } from '@/lib/server/auth/auth';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 import {
   getDocumentRange,
   isMissingBlobError as isMissingDocumentBlobError,
@@ -183,6 +184,9 @@ export async function GET(req: NextRequest) {
       event: 'documents.preview.fallback.failed',
       error: errorToLog(error),
     }, 'Failed to load document preview fallback');
-    return NextResponse.json({ error: 'Failed to load document preview' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to load document preview',
+      normalize: { code: 'DOCUMENTS_PREVIEW_FALLBACK_FAILED', errorClass: 'storage' },
+    });
   }
 }
diff --git a/src/app/api/documents/blob/preview/presign/route.ts b/src/app/api/documents/blob/preview/presign/route.ts
index 8c98c49..717902e 100644
--- a/src/app/api/documents/blob/preview/presign/route.ts
+++ b/src/app/api/documents/blob/preview/presign/route.ts
@@ -3,6 +3,7 @@ import { presignDocumentPreviewGet } from '@/lib/server/documents/previews-blobs
 import { ensureDocumentPreview } from '@/lib/server/documents/previews';
 import { validatePreviewRequest } from '../utils';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 
 export const dynamic = 'force-dynamic';
 
@@ -59,6 +60,9 @@ export async function GET(req: NextRequest) {
       event: 'documents.preview.presign.failed',
       error: errorToLog(error),
     }, 'Failed to create document preview signature');
-    return NextResponse.json({ error: 'Failed to prepare document preview' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to prepare document preview',
+      normalize: { code: 'DOCUMENTS_PREVIEW_PRESIGN_FAILED', errorClass: 'storage' },
+    });
   }
 }
diff --git a/src/app/api/documents/blob/upload/fallback/route.ts b/src/app/api/documents/blob/upload/fallback/route.ts
index c3d7a9f..bc188df 100644
--- a/src/app/api/documents/blob/upload/fallback/route.ts
+++ b/src/app/api/documents/blob/upload/fallback/route.ts
@@ -4,6 +4,7 @@ import { isValidDocumentId, putDocumentBlob } from '@/lib/server/documents/blobs
 import { isS3Configured } from '@/lib/server/storage/s3';
 import { getOpenReaderTestNamespace } from '@/lib/server/testing/test-namespace';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 
 export const dynamic = 'force-dynamic';
 
@@ -58,6 +59,9 @@ export async function PUT(req: NextRequest) {
       event: 'documents.blob.upload.fallback.failed',
       error: errorToLog(error),
     }, 'Failed to proxy-upload document blob');
-    return NextResponse.json({ error: 'Failed to upload document blob' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to upload document blob',
+      normalize: { code: 'DOCUMENTS_BLOB_UPLOAD_FALLBACK_FAILED', errorClass: 'storage' },
+    });
   }
 }
diff --git a/src/app/api/documents/blob/upload/presign/route.ts b/src/app/api/documents/blob/upload/presign/route.ts
index a05e6eb..07756aa 100644
--- a/src/app/api/documents/blob/upload/presign/route.ts
+++ b/src/app/api/documents/blob/upload/presign/route.ts
@@ -4,6 +4,7 @@ import { isValidDocumentId, presignPut } from '@/lib/server/documents/blobstore'
 import { getOpenReaderTestNamespace } from '@/lib/server/testing/test-namespace';
 import { isS3Configured } from '@/lib/server/storage/s3';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 
 export const dynamic = 'force-dynamic';
 
@@ -70,6 +71,9 @@ export async function POST(req: NextRequest) {
       event: 'documents.blob.upload.presign.failed',
       error: errorToLog(error),
     }, 'Failed to create document upload signatures');
-    return NextResponse.json({ error: 'Failed to presign uploads' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to presign uploads',
+      normalize: { code: 'DOCUMENTS_BLOB_UPLOAD_PRESIGN_FAILED', errorClass: 'storage' },
+    });
   }
 }
diff --git a/src/app/api/documents/docx-to-pdf/upload/route.ts b/src/app/api/documents/docx-to-pdf/upload/route.ts
index e427c78..91866af 100644
--- a/src/app/api/documents/docx-to-pdf/upload/route.ts
+++ b/src/app/api/documents/docx-to-pdf/upload/route.ts
@@ -16,6 +16,7 @@ import { getOpenReaderTestNamespace, getUnclaimedUserIdForNamespace } from '@/li
 import { isS3Configured } from '@/lib/server/storage/s3';
 import { putDocumentBlob } from '@/lib/server/documents/blobstore';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 
 const DOCSTORE_DIR = path.join(process.cwd(), 'docstore');
 const TEMP_DIR = path.join(DOCSTORE_DIR, 'tmp');
@@ -195,6 +196,9 @@ export async function POST(req: NextRequest) {
       event: 'documents.docx.convert_upload.failed',
       error: errorToLog(error),
     }, 'Failed converting/uploading DOCX');
-    return NextResponse.json({ error: 'Failed to convert document' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to convert document',
+      normalize: { code: 'DOCUMENTS_DOCX_CONVERT_UPLOAD_FAILED', errorClass: 'upstream' },
+    });
   }
 }
diff --git a/src/app/api/documents/library/route.ts b/src/app/api/documents/library/route.ts
index f66686c..44c8e32 100644
--- a/src/app/api/documents/library/route.ts
+++ b/src/app/api/documents/library/route.ts
@@ -6,6 +6,7 @@ import { parseLibraryRoots } from '@/lib/server/storage/library-mount';
 import type { DocumentType } from '@/types/documents';
 import { auth } from '@/lib/server/auth/auth';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 
 export const dynamic = 'force-dynamic';
 
@@ -116,7 +117,10 @@ export async function GET(req: NextRequest) {
       event: 'documents.library.auth_check.failed',
       error: errorToLog(error),
     }, 'Failed to check auth for library route');
-    return NextResponse.json({ error: 'Error checking auth' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Error checking auth',
+      normalize: { code: 'DOCUMENTS_LIBRARY_AUTH_CHECK_FAILED', errorClass: 'auth' },
+    });
   }
 
   const url = new URL(req.url);
diff --git a/src/app/api/documents/route.ts b/src/app/api/documents/route.ts
index 141812b..c56891c 100644
--- a/src/app/api/documents/route.ts
+++ b/src/app/api/documents/route.ts
@@ -5,6 +5,7 @@ import { documents } from '@/db/schema';
 import { requireAuthContext } from '@/lib/server/auth/auth';
 import { safeDocumentName, toDocumentTypeFromName } from '@/lib/server/documents/utils';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 import {
   cleanupDocumentPreviewArtifacts,
   deleteDocumentPreviewRows,
@@ -197,7 +198,10 @@ export async function POST(req: NextRequest) {
       event: 'documents.register.failed',
       error: errorToLog(error),
     }, 'Failed to register documents');
-    return NextResponse.json({ error: 'Failed to register documents' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to register documents',
+      normalize: { code: 'DOCUMENTS_REGISTER_FAILED', errorClass: 'db' },
+    });
   }
 }
 
@@ -276,7 +280,10 @@ export async function GET(req: NextRequest) {
       event: 'documents.list.failed',
       error: errorToLog(error),
     }, 'Failed to load document metadata');
-    return NextResponse.json({ error: 'Failed to load documents' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to load documents',
+      normalize: { code: 'DOCUMENTS_LIST_FAILED', errorClass: 'db' },
+    });
   }
 }
 
@@ -391,6 +398,9 @@ export async function DELETE(req: NextRequest) {
       event: 'documents.delete.failed',
       error: errorToLog(error),
     }, 'Failed to delete documents');
-    return NextResponse.json({ error: 'Failed to delete documents' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to delete documents',
+      normalize: { code: 'DOCUMENTS_DELETE_FAILED', errorClass: 'db' },
+    });
   }
 }
diff --git a/src/app/api/rate-limit/status/route.ts b/src/app/api/rate-limit/status/route.ts
index e8d3bbc..e541f9f 100644
--- a/src/app/api/rate-limit/status/route.ts
+++ b/src/app/api/rate-limit/status/route.ts
@@ -7,6 +7,7 @@ import { getClientIp } from '@/lib/server/rate-limit/request-ip';
 import { getOrCreateDeviceId, setDeviceIdCookie } from '@/lib/server/rate-limit/device-id';
 import { nextUtcMidnightTimestampMs } from '@/lib/shared/timestamps';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 
 export const dynamic = 'force-dynamic';
 
@@ -89,9 +90,9 @@ export async function GET(req: NextRequest) {
       event: 'rate_limit.status.get.failed',
       error: errorToLog(error),
     }, 'Failed to get rate limit status');
-    return NextResponse.json(
-      { error: 'Failed to get rate limit status' },
-      { status: 500 }
-    );
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to get rate limit status',
+      normalize: { code: 'RATE_LIMIT_STATUS_GET_FAILED', errorClass: 'db' },
+    });
   }
 }
diff --git a/src/app/api/tts/segments/audio/fallback/route.ts b/src/app/api/tts/segments/audio/fallback/route.ts
index 159713d..22a3120 100644
--- a/src/app/api/tts/segments/audio/fallback/route.ts
+++ b/src/app/api/tts/segments/audio/fallback/route.ts
@@ -7,7 +7,8 @@ import {
   ttsSegmentsS3NotConfiguredResponse,
 } from '@/lib/server/tts/segments-audio';
 import { getTtsSegmentAudioObjectStream } from '@/lib/server/tts/segments-blobstore';
-import { createRequestLogger, errorToLog } from '@/lib/server/logger';
+import { createRequestLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 
 export const dynamic = 'force-dynamic';
 
@@ -50,10 +51,12 @@ export async function GET(request: NextRequest) {
       headers,
     });
   } catch (error) {
-    logger.error({
+    return errorResponse(error, {
+      logger,
       event: 'tts.segments.audio_fallback_failed',
-      error: errorToLog(error),
-    }, 'Failed to load segment audio from fallback route');
-    return NextResponse.json({ error: 'Failed to load segment audio' }, { status: 500 });
+      msg: 'Failed to load segment audio from fallback route',
+      apiErrorMessage: 'Failed to load segment audio',
+      normalize: { code: 'TTS_SEGMENT_AUDIO_FALLBACK_FAILED', errorClass: 'storage' },
+    });
   }
 }
diff --git a/src/app/api/tts/segments/clear/route.ts b/src/app/api/tts/segments/clear/route.ts
index c794bbf..bcbca11 100644
--- a/src/app/api/tts/segments/clear/route.ts
+++ b/src/app/api/tts/segments/clear/route.ts
@@ -1,7 +1,8 @@
 import { NextRequest, NextResponse } from 'next/server';
 import { resolveSegmentDocumentScope } from '@/lib/server/tts/segments-auth';
 import { clearTtsSegmentCache } from '@/lib/server/tts/segments-cache';
-import { createRequestLogger, errorToLog } from '@/lib/server/logger';
+import { createRequestLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 
 export const runtime = 'nodejs';
 export const dynamic = 'force-dynamic';
@@ -39,10 +40,12 @@ export async function POST(request: NextRequest) {
       ...cleared,
     });
   } catch (error) {
-    logger.error({
+    return errorResponse(error, {
+      logger,
       event: 'tts.segments.clear_failed',
-      error: errorToLog(error),
-    }, 'Failed to clear TTS segment cache');
-    return NextResponse.json({ error: 'Failed to clear TTS segment cache' }, { status: 500 });
+      msg: 'Failed to clear TTS segment cache',
+      apiErrorMessage: 'Failed to clear TTS segment cache',
+      normalize: { code: 'TTS_SEGMENTS_CLEAR_FAILED', errorClass: 'storage' },
+    });
   }
 }
diff --git a/src/app/api/tts/segments/ensure/route.ts b/src/app/api/tts/segments/ensure/route.ts
index 7e4f48f..a3dc204 100644
--- a/src/app/api/tts/segments/ensure/route.ts
+++ b/src/app/api/tts/segments/ensure/route.ts
@@ -35,6 +35,7 @@ import { getResolvedRuntimeConfig } from '@/lib/server/runtime-config';
 import { resolveTtsModelForProvider } from '@/lib/shared/tts-provider-policy';
 import { resolveSegmentAudioUrls } from '@/lib/server/tts/segment-audio-urls';
 import { createRequestLogger, errorToLog } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 import type {
   TTSSegmentInput,
   TTSSegmentManifestItem,
@@ -828,11 +829,13 @@ export async function POST(request: NextRequest) {
     attachDeviceIdCookie(response, deviceIdToSet, didCreateDeviceIdCookie);
     return response;
   } catch (error) {
-    logger.error({
+    const response = errorResponse(error, {
+      logger,
       event: 'tts.segments.ensure.route_failed',
-      error: errorToLog(error),
-    }, 'TTS segments ensure route failed');
-    const response = NextResponse.json({ error: 'Failed to ensure TTS segments' }, { status: 500 });
+      msg: 'TTS segments ensure route failed',
+      apiErrorMessage: 'Failed to ensure TTS segments',
+      normalize: { code: 'TTS_SEGMENTS_ENSURE_ROUTE_FAILED', errorClass: 'upstream' },
+    });
     attachDeviceIdCookie(response, deviceIdToSet, didCreateDeviceIdCookie);
     return response;
   }
diff --git a/src/app/api/tts/segments/manifest/route.ts b/src/app/api/tts/segments/manifest/route.ts
index b8cad65..c78e602 100644
--- a/src/app/api/tts/segments/manifest/route.ts
+++ b/src/app/api/tts/segments/manifest/route.ts
@@ -20,7 +20,8 @@ import type {
 import { isTtsProviderType } from '@/lib/shared/tts-provider-catalog';
 import { resolveEffectiveProviderType } from '@/lib/shared/tts-provider-policy';
 import { resolveSegmentAudioUrls } from '@/lib/server/tts/segment-audio-urls';
-import { createRequestLogger, errorToLog } from '@/lib/server/logger';
+import { createRequestLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 
 export const runtime = 'nodejs';
 export const dynamic = 'force-dynamic';
@@ -386,10 +387,12 @@ export async function GET(request: NextRequest) {
     };
     return NextResponse.json(response);
   } catch (error) {
-    logger.error({
+    return errorResponse(error, {
+      logger,
       event: 'tts.segments.manifest.list_failed',
-      error: errorToLog(error),
-    }, 'Failed to list TTS segments');
-    return NextResponse.json({ error: 'Failed to list TTS segments' }, { status: 500 });
+      msg: 'Failed to list TTS segments',
+      apiErrorMessage: 'Failed to list TTS segments',
+      normalize: { code: 'TTS_SEGMENTS_MANIFEST_LIST_FAILED', errorClass: 'db' },
+    });
   }
 }
diff --git a/src/app/api/tts/voices/route.ts b/src/app/api/tts/voices/route.ts
index f024431..0e1b5f5 100644
--- a/src/app/api/tts/voices/route.ts
+++ b/src/app/api/tts/voices/route.ts
@@ -6,6 +6,8 @@ import { resolveVoices } from '@/lib/server/tts/voice-resolution';
 import { resolveTtsCredentials } from '@/lib/server/admin/resolve-credentials';
 import { getResolvedRuntimeConfig } from '@/lib/server/runtime-config';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
+import { normalizeServerError, toApiErrorBody, toHttpStatus } from '@/lib/server/errors/contract';
 
 export async function GET(req: NextRequest) {
   try {
@@ -41,7 +43,10 @@ export async function GET(req: NextRequest) {
     }
 
     if (!isBuiltInTtsProviderId(resolved.provider)) {
-      return NextResponse.json({ error: `Unsupported provider type: ${resolved.provider}` }, { status: 500 });
+      return errorResponse(new Error(`Unsupported provider type: ${resolved.provider}`), {
+        apiErrorMessage: `Unsupported provider type: ${resolved.provider}`,
+        normalize: { code: 'TTS_VOICES_UNSUPPORTED_PROVIDER', errorClass: 'validation', httpStatus: 500 },
+      });
     }
     const effectiveProviderRef = resolved.adminRecord?.slug
       ?? req.headers.get('x-tts-provider')
@@ -69,16 +74,24 @@ export async function GET(req: NextRequest) {
     const providerRef = req.headers.get('x-tts-provider') || 'openai';
     const model = req.headers.get('x-tts-model') || 'tts-1';
     const provider = isBuiltInTtsProviderId(providerRef) ? providerRef : 'openai';
+    const normalized = normalizeServerError(error, {
+      code: 'TTS_VOICES_RESOLVE_FAILED',
+      errorClass: 'upstream',
+    });
+    const fallbackVoices = resolveTtsProviderModelPolicy({
+      providerRef,
+      providerType: provider,
+      model,
+    }).defaultVoices;
     return NextResponse.json(
       {
-        error: 'Failed to resolve voices',
-        fallbackVoices: resolveTtsProviderModelPolicy({
-          providerRef,
-          providerType: provider,
-          model,
-        }).defaultVoices,
+        ...toApiErrorBody(
+          { ...normalized, message: 'Failed to resolve voices' },
+          { includeDetails: false, includeRetryable: false },
+        ),
+        fallbackVoices,
       },
-      { status: 500 },
+      { status: toHttpStatus(normalized) },
     );
   }
 }
diff --git a/src/app/api/user/claim/route.ts b/src/app/api/user/claim/route.ts
index d299d1b..14cd313 100644
--- a/src/app/api/user/claim/route.ts
+++ b/src/app/api/user/claim/route.ts
@@ -6,6 +6,7 @@ import { audiobooks, documents, userDocumentProgress, userPreferences } from '@/
 import { count, eq, ne } from 'drizzle-orm';
 import { getOpenReaderTestNamespace, getUnclaimedUserIdForNamespace } from '@/lib/server/testing/test-namespace';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 
 async function checkClaimMigrationReadiness(): Promise<NextResponse | null> {
   const [legacyRows] = await db
@@ -61,7 +62,10 @@ export async function GET(req: NextRequest) {
       event: 'user.claim.status.failed',
       error: errorToLog(error),
     }, 'Failed checking claimable data');
-    return NextResponse.json({ error: 'Internal Server Error' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Internal Server Error',
+      normalize: { code: 'USER_CLAIM_STATUS_FAILED', errorClass: 'db' },
+    });
   }
 }
 
@@ -91,6 +95,9 @@ export async function POST(req: NextRequest) {
       event: 'user.claim.execute.failed',
       error: errorToLog(error),
     }, 'Failed claiming anonymous data');
-    return NextResponse.json({ error: 'Internal Server Error' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Internal Server Error',
+      normalize: { code: 'USER_CLAIM_EXECUTE_FAILED', errorClass: 'db' },
+    });
   }
 }
diff --git a/src/app/api/user/export/route.ts b/src/app/api/user/export/route.ts
index 98e7552..66920bf 100644
--- a/src/app/api/user/export/route.ts
+++ b/src/app/api/user/export/route.ts
@@ -12,6 +12,7 @@ import { isS3Configured } from '@/lib/server/storage/s3';
 import { getOpenReaderTestNamespace } from '@/lib/server/testing/test-namespace';
 import { nowTimestampMs } from '@/lib/shared/timestamps';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 
 export const dynamic = 'force-dynamic';
 
@@ -24,7 +25,10 @@ export async function GET(req: NextRequest) {
   }
 
   if (!auth) {
-    return NextResponse.json({ error: 'Auth not initialized' }, { status: 500 });
+    return errorResponse(new Error('Auth not initialized'), {
+      apiErrorMessage: 'Auth not initialized',
+      normalize: { code: 'USER_EXPORT_AUTH_NOT_INITIALIZED', errorClass: 'auth', httpStatus: 500 },
+    });
   }
 
   const session = await auth.api.getSession({
diff --git a/src/app/api/user/state/changelog/version-check/route.ts b/src/app/api/user/state/changelog/version-check/route.ts
index 47c7adc..b36d74f 100644
--- a/src/app/api/user/state/changelog/version-check/route.ts
+++ b/src/app/api/user/state/changelog/version-check/route.ts
@@ -6,6 +6,7 @@ import { normalizeVersion, shouldOpenChangelogForVersionChange } from '@/lib/sha
 import { nowTimestampMs } from '@/lib/shared/timestamps';
 import { resolveUserStateScope } from '@/lib/server/user/resolve-state-scope';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 import {
   deserializeUserPreferencesPayload,
   extractUserPreferencesMeta,
@@ -85,6 +86,9 @@ export async function POST(req: NextRequest) {
       event: 'user.changelog.version_check.failed',
       error: errorToLog(error),
     }, 'Failed to check changelog version');
-    return NextResponse.json({ error: 'Failed to check changelog version' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to check changelog version',
+      normalize: { code: 'USER_CHANGELOG_VERSION_CHECK_FAILED', errorClass: 'db' },
+    });
   }
 }
diff --git a/src/app/api/user/state/preferences/route.ts b/src/app/api/user/state/preferences/route.ts
index 19f10b4..78f38ee 100644
--- a/src/app/api/user/state/preferences/route.ts
+++ b/src/app/api/user/state/preferences/route.ts
@@ -10,6 +10,7 @@ import { listAdminProviders } from '@/lib/server/admin/providers';
 import { getResolvedRuntimeConfig } from '@/lib/server/runtime-config';
 import { normalizeLegacyProviderRef, resolveProviderDefaults } from '@/lib/shared/tts-provider-policy';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 import {
   deserializeUserPreferencesPayload,
   extractUserPreferencesMeta,
@@ -246,7 +247,10 @@ export async function GET(req: NextRequest) {
       event: 'user.preferences.load.failed',
       error: errorToLog(error),
     }, 'Failed to load user preferences');
-    return NextResponse.json({ error: 'Failed to load user preferences' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to load user preferences',
+      normalize: { code: 'USER_PREFERENCES_LOAD_FAILED', errorClass: 'db' },
+    });
   }
 }
 
@@ -323,6 +327,9 @@ export async function PUT(req: NextRequest) {
       event: 'user.preferences.update.failed',
       error: errorToLog(error),
     }, 'Failed to update user preferences');
-    return NextResponse.json({ error: 'Failed to update user preferences' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to update user preferences',
+      normalize: { code: 'USER_PREFERENCES_UPDATE_FAILED', errorClass: 'db' },
+    });
   }
 }
diff --git a/src/app/api/user/state/progress/route.ts b/src/app/api/user/state/progress/route.ts
index 7e4d377..1fef9f5 100644
--- a/src/app/api/user/state/progress/route.ts
+++ b/src/app/api/user/state/progress/route.ts
@@ -7,6 +7,7 @@ import { isValidDocumentId } from '@/lib/server/documents/blobstore';
 import { resolveUserStateScope } from '@/lib/server/user/resolve-state-scope';
 import { coerceTimestampMs, nowTimestampMs } from '@/lib/shared/timestamps';
 import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { errorResponse } from '@/lib/server/errors/next-response';
 
 export const dynamic = 'force-dynamic';
 
@@ -67,7 +68,10 @@ export async function GET(req: NextRequest) {
       event: 'user.progress.load.failed',
       error: errorToLog(error),
     }, 'Failed to load user progress');
-    return NextResponse.json({ error: 'Failed to load user progress' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to load user progress',
+      normalize: { code: 'USER_PROGRESS_LOAD_FAILED', errorClass: 'db' },
+    });
   }
 }
 
@@ -180,6 +184,9 @@ export async function PUT(req: NextRequest) {
       event: 'user.progress.update.failed',
       error: errorToLog(error),
     }, 'Failed to update user progress');
-    return NextResponse.json({ error: 'Failed to update user progress' }, { status: 500 });
+    return errorResponse(error, {
+      apiErrorMessage: 'Failed to update user progress',
+      normalize: { code: 'USER_PROGRESS_UPDATE_FAILED', errorClass: 'db' },
+    });
   }
 }
diff --git a/src/lib/server/compute/worker-op-state.ts b/src/lib/server/compute/worker-op-state.ts
index 7cfe88b..dc36e1e 100644
--- a/src/lib/server/compute/worker-op-state.ts
+++ b/src/lib/server/compute/worker-op-state.ts
@@ -1,6 +1,7 @@
 import { getWorkerClientConfigFromEnv } from '@/lib/server/compute/worker';
 import type { WorkerOperationState } from '@openreader/compute-core/api-contracts';
-import { errorToLog, serverLogger } from '@/lib/server/logger';
+import { serverLogger } from '@/lib/server/logger';
+import { logDegraded } from '@/lib/server/errors/logging';
 
 const WORKER_OP_REQUEST_TIMEOUT_MS = 2_500;
 
@@ -14,13 +15,13 @@ export async function fetchWorkerOperationState<Result>(
   try {
     cfg = getWorkerClientConfigFromEnv();
   } catch (error) {
-    serverLogger.warn({
+    logDegraded(serverLogger, {
       event: 'compute.worker_op_state.config.invalid',
-      opId: normalized,
-      degraded: true,
+      msg: 'Worker client env missing/invalid',
       step: 'read_worker_config',
-      error: errorToLog(error),
-    }, 'Worker client env missing/invalid');
+      context: { opId: normalized },
+      error,
+    });
     return null;
   }
 
@@ -40,39 +41,41 @@ export async function fetchWorkerOperationState<Result>(
 
     if (!res.ok) {
       const upstreamResponseBody = await res.text().catch(() => '');
-      serverLogger.warn({
+      logDegraded(serverLogger, {
         event: 'compute.worker_op_state.fetch.failed',
-        opId: normalized,
-        status: res.status,
-        upstreamResponseBody,
-        degraded: true,
+        msg: 'Worker op request failed',
         step: 'fetch_worker_op',
+        context: {
+          opId: normalized,
+          status: res.status,
+          upstreamResponseBody,
+        },
         error: {
           name: 'WorkerOpStateFetchFailed',
           message: `Worker op request failed with status ${res.status}`,
         },
-      }, 'Worker op request failed');
+      });
       return null;
     }
     const parsed = await res.json() as WorkerOperationState<Result>;
     if (!parsed || typeof parsed !== 'object' || parsed.opId !== normalized) {
-      serverLogger.warn({
+      logDegraded(serverLogger, {
         event: 'compute.worker_op_state.response.invalid',
-        opId: normalized,
-        degraded: true,
+        msg: 'Worker op response invalid',
         step: 'validate_worker_op_response',
-      }, 'Worker op response invalid');
+        context: { opId: normalized },
+      });
       return null;
     }
     return parsed;
   } catch (error) {
-    serverLogger.warn({
+    logDegraded(serverLogger, {
       event: 'compute.worker_op_state.fetch.error',
-      opId: normalized,
-      degraded: true,
+      msg: 'Worker op request threw',
       step: 'fetch_worker_op',
-      error: errorToLog(error),
-    }, 'Worker op request threw');
+      context: { opId: normalized },
+      error,
+    });
     return null;
   } finally {
     clearTimeout(timeout);
diff --git a/src/lib/server/errors/catalog.ts b/src/lib/server/errors/catalog.ts
new file mode 100644
index 0000000..c7febb0
--- /dev/null
+++ b/src/lib/server/errors/catalog.ts
@@ -0,0 +1,65 @@
+import type { ServerErrorClass } from '@/lib/server/errors/contract';
+
+export type ServerErrorCatalogEntry = {
+  code: string;
+  errorClass: ServerErrorClass;
+  httpStatus: number;
+  retryable: boolean;
+  ownerDomain: string;
+  description: string;
+};
+
+export const SERVER_ERROR_CATALOG: ServerErrorCatalogEntry[] = [
+  {
+    code: 'UPSTREAM_RATE_LIMIT',
+    errorClass: 'upstream',
+    httpStatus: 429,
+    retryable: true,
+    ownerDomain: 'tts',
+    description: 'Upstream provider rate limited the request.',
+  },
+  {
+    code: 'UPSTREAM_TTS_ERROR',
+    errorClass: 'upstream',
+    httpStatus: 502,
+    retryable: true,
+    ownerDomain: 'tts',
+    description: 'Upstream TTS provider returned a 5xx/transport failure.',
+  },
+  {
+    code: 'DOCUMENTS_BLOB_MISSING',
+    errorClass: 'storage',
+    httpStatus: 404,
+    retryable: false,
+    ownerDomain: 'documents',
+    description: 'Document blob was not found in backing storage.',
+  },
+  {
+    code: 'COMPUTE_WORKER_UNAVAILABLE',
+    errorClass: 'upstream',
+    httpStatus: 503,
+    retryable: true,
+    ownerDomain: 'compute',
+    description: 'Compute worker endpoint unavailable or misconfigured.',
+  },
+  {
+    code: 'AUTH_UNAUTHORIZED',
+    errorClass: 'auth',
+    httpStatus: 401,
+    retryable: false,
+    ownerDomain: 'auth',
+    description: 'Request requires authentication and no valid session was present.',
+  },
+  {
+    code: 'UNKNOWN_SERVER_ERROR',
+    errorClass: 'unknown',
+    httpStatus: 500,
+    retryable: false,
+    ownerDomain: 'server',
+    description: 'Fallback classification for uncategorized server exceptions.',
+  },
+];
+
+export const SERVER_ERROR_CATALOG_BY_CODE = new Map(
+  SERVER_ERROR_CATALOG.map((entry) => [entry.code, entry] as const),
+);
diff --git a/src/lib/server/errors/contract.ts b/src/lib/server/errors/contract.ts
new file mode 100644
index 0000000..6917bd1
--- /dev/null
+++ b/src/lib/server/errors/contract.ts
@@ -0,0 +1,203 @@
+export type ServerErrorClass =
+  | 'validation'
+  | 'auth'
+  | 'permission'
+  | 'upstream'
+  | 'storage'
+  | 'db'
+  | 'timeout'
+  | 'unknown';
+
+type ServerErrorDefaults = {
+  httpStatus: number;
+  retryable: boolean;
+};
+
+const CLASS_DEFAULTS: Record<ServerErrorClass, ServerErrorDefaults> = {
+  validation: { httpStatus: 400, retryable: false },
+  auth: { httpStatus: 401, retryable: false },
+  permission: { httpStatus: 403, retryable: false },
+  upstream: { httpStatus: 502, retryable: true },
+  storage: { httpStatus: 503, retryable: true },
+  db: { httpStatus: 500, retryable: true },
+  timeout: { httpStatus: 504, retryable: true },
+  unknown: { httpStatus: 500, retryable: false },
+};
+
+const ERROR_CODE_PATTERN = /^[A-Z][A-Z0-9_]{2,}$/;
+
+export type ServerErrorContext = {
+  code?: string;
+  message?: string;
+  errorClass?: ServerErrorClass;
+  httpStatus?: number;
+  retryable?: boolean;
+  details?: Record<string, unknown>;
+};
+
+export class ServerAppError extends Error {
+  readonly code: string;
+  readonly errorClass: ServerErrorClass;
+  readonly httpStatus: number;
+  readonly retryable: boolean;
+  readonly details?: Record<string, unknown>;
+
+  constructor(input: {
+    code: string;
+    message: string;
+    errorClass: ServerErrorClass;
+    httpStatus?: number;
+    retryable?: boolean;
+    details?: Record<string, unknown>;
+    cause?: unknown;
+  }) {
+    super(input.message, input.cause !== undefined ? { cause: input.cause } : undefined);
+    this.name = 'ServerAppError';
+    this.code = input.code;
+    this.errorClass = input.errorClass;
+    const defaults = CLASS_DEFAULTS[input.errorClass];
+    this.httpStatus = input.httpStatus ?? defaults.httpStatus;
+    this.retryable = input.retryable ?? defaults.retryable;
+    if (input.details) this.details = input.details;
+  }
+}
+
+export function isServerAppError(value: unknown): value is ServerAppError {
+  return value instanceof ServerAppError;
+}
+
+function normalizeFromErrorInstance(error: Error): {
+  message: string;
+  code?: string;
+  errorClass?: ServerErrorClass;
+  httpStatus?: number;
+  retryable?: boolean;
+  details?: Record<string, unknown>;
+} {
+  const rec = error as Error & {
+    code?: unknown;
+    errorClass?: unknown;
+    httpStatus?: unknown;
+    retryable?: unknown;
+    details?: unknown;
+  };
+  const code = typeof rec.code === 'string' && rec.code.trim() ? rec.code.trim() : undefined;
+  const errorClass = typeof rec.errorClass === 'string' && rec.errorClass in CLASS_DEFAULTS
+    ? rec.errorClass as ServerErrorClass
+    : undefined;
+  const httpStatus = typeof rec.httpStatus === 'number' && Number.isFinite(rec.httpStatus)
+    ? Math.max(400, Math.min(599, Math.floor(rec.httpStatus)))
+    : undefined;
+  const retryable = typeof rec.retryable === 'boolean' ? rec.retryable : undefined;
+  const details = rec.details && typeof rec.details === 'object'
+    ? rec.details as Record<string, unknown>
+    : undefined;
+  return {
+    message: error.message || 'Internal server error',
+    ...(code ? { code } : {}),
+    ...(errorClass ? { errorClass } : {}),
+    ...(httpStatus ? { httpStatus } : {}),
+    ...(typeof retryable === 'boolean' ? { retryable } : {}),
+    ...(details ? { details } : {}),
+  };
+}
+
+export type NormalizedServerError = {
+  code: string;
+  message: string;
+  errorClass: ServerErrorClass;
+  httpStatus: number;
+  retryable: boolean;
+  details?: Record<string, unknown>;
+  cause?: unknown;
+};
+
+export function normalizeServerError(
+  error: unknown,
+  ctx: ServerErrorContext = {},
+): NormalizedServerError {
+  if (isServerAppError(error)) {
+    return {
+      code: error.code,
+      message: error.message,
+      errorClass: error.errorClass,
+      httpStatus: error.httpStatus,
+      retryable: error.retryable,
+      ...(error.details ? { details: error.details } : {}),
+      ...(error.cause !== undefined ? { cause: error.cause } : {}),
+    };
+  }
+
+  const normalizedFromThrowable = error instanceof Error ? normalizeFromErrorInstance(error) : {
+    message: String(error),
+  };
+
+  const errorClass = ctx.errorClass
+    ?? normalizedFromThrowable.errorClass
+    ?? 'unknown';
+  const defaults = CLASS_DEFAULTS[errorClass];
+
+  const rawCode = ctx.code
+      ?? normalizedFromThrowable.code
+      ?? 'UNKNOWN_SERVER_ERROR';
+  const code = ERROR_CODE_PATTERN.test(rawCode) ? rawCode : 'UNKNOWN_SERVER_ERROR';
+
+  return {
+    code,
+    message: ctx.message
+      ?? normalizedFromThrowable.message
+      ?? 'Internal server error',
+    errorClass,
+    httpStatus: ctx.httpStatus
+      ?? normalizedFromThrowable.httpStatus
+      ?? defaults.httpStatus,
+    retryable: ctx.retryable
+      ?? normalizedFromThrowable.retryable
+      ?? defaults.retryable,
+    ...(ctx.details ? { details: ctx.details } : normalizedFromThrowable.details ? { details: normalizedFromThrowable.details } : {}),
+    ...(error instanceof Error && error.cause !== undefined ? { cause: error.cause } : {}),
+  };
+}
+
+export function toApiErrorBody(
+  input: NormalizedServerError,
+  options: {
+    includeErrorCode?: boolean;
+    includeRetryable?: boolean;
+    includeDetails?: boolean;
+  } = {},
+): {
+  error: string;
+  errorCode?: string;
+  retryable?: boolean;
+  details?: Record<string, unknown>;
+} {
+  const includeErrorCode = options.includeErrorCode ?? true;
+  const includeRetryable = options.includeRetryable ?? true;
+  const includeDetails = options.includeDetails ?? true;
+  return {
+    error: input.message,
+    ...(includeErrorCode ? { errorCode: input.code } : {}),
+    ...(includeRetryable ? { retryable: input.retryable } : {}),
+    ...(includeDetails && input.details ? { details: input.details } : {}),
+  };
+}
+
+export function toHttpStatus(input: NormalizedServerError): number {
+  return input.httpStatus;
+}
+
+export function createServerAppError(input: {
+  code: string;
+  message: string;
+  errorClass: ServerErrorClass;
+  httpStatus?: number;
+  retryable?: boolean;
+  details?: Record<string, unknown>;
+  cause?: unknown;
+}): ServerAppError {
+  if (!ERROR_CODE_PATTERN.test(input.code)) {
+    throw new Error(`Invalid ServerAppError code "${input.code}" (expected ${ERROR_CODE_PATTERN.toString()})`);
+  }
+  return new ServerAppError(input);
+}
diff --git a/src/lib/server/errors/logging.ts b/src/lib/server/errors/logging.ts
new file mode 100644
index 0000000..0785134
--- /dev/null
+++ b/src/lib/server/errors/logging.ts
@@ -0,0 +1,48 @@
+import type { ServerLogger } from '@/lib/server/logger';
+import { errorToLog } from '@/lib/server/logger';
+import { normalizeServerError, type NormalizedServerError, type ServerErrorContext } from '@/lib/server/errors/contract';
+
+type LogContext = Record<string, unknown>;
+
+export function logServerError(
+  logger: ServerLogger,
+  input: {
+    event: string;
+    error: unknown;
+    msg: string;
+    context?: LogContext;
+    normalize?: ServerErrorContext;
+  },
+): NormalizedServerError {
+  const normalized = normalizeServerError(input.error, input.normalize);
+  logger.error({
+    event: input.event,
+    ...(input.context || {}),
+    error: {
+      ...errorToLog(input.error),
+      code: normalized.code,
+    },
+  }, `${input.msg}`);
+  return normalized;
+}
+
+export function logDegraded(
+  logger: ServerLogger,
+  input: {
+    event: string;
+    msg: string;
+    step?: string;
+    fallbackPath?: string;
+    context?: LogContext;
+    error?: unknown;
+  },
+): void {
+  logger.warn({
+    event: input.event,
+    degraded: true,
+    ...(input.step ? { step: input.step } : {}),
+    ...(input.fallbackPath ? { fallbackPath: input.fallbackPath } : {}),
+    ...(input.context || {}),
+    ...(input.error !== undefined ? { error: errorToLog(input.error) } : {}),
+  }, `${input.msg}`);
+}
diff --git a/src/lib/server/errors/next-response.ts b/src/lib/server/errors/next-response.ts
new file mode 100644
index 0000000..599d8c0
--- /dev/null
+++ b/src/lib/server/errors/next-response.ts
@@ -0,0 +1,90 @@
+import type { NextRequest } from 'next/server';
+import { NextResponse } from 'next/server';
+import type { ServerLogger } from '@/lib/server/logger';
+import { normalizeServerError, toApiErrorBody, toHttpStatus, type NormalizedServerError, type ServerErrorContext } from '@/lib/server/errors/contract';
+import { logServerError } from '@/lib/server/errors/logging';
+
+export type ErrorResponseOptions = {
+  logger?: ServerLogger;
+  event?: string;
+  msg?: string;
+  normalize?: ServerErrorContext;
+  apiErrorMessage?: string;
+  includeErrorCode?: boolean;
+  includeRetryable?: boolean;
+  includeDetails?: boolean;
+};
+
+export function errorResponse(
+  error: unknown,
+  options: ErrorResponseOptions = {},
+): NextResponse {
+  const normalized = normalizeServerError(error, options.normalize);
+  if (options.logger && options.event && options.msg) {
+    logServerError(options.logger, {
+      event: options.event,
+      error,
+      msg: options.msg,
+      context: {},
+      normalize: options.normalize,
+    });
+  }
+  const apiError = options.apiErrorMessage ?? normalized.message;
+  const body = toApiErrorBody(
+    { ...normalized, message: apiError },
+    {
+      includeErrorCode: options.includeErrorCode ?? true,
+      includeRetryable: options.includeRetryable ?? true,
+      includeDetails: options.includeDetails ?? false,
+    },
+  );
+  return NextResponse.json(body, { status: toHttpStatus(normalized) });
+}
+
+export type ErrorBoundaryContext = {
+  route: string;
+  logger: ServerLogger;
+  event: string;
+  msg: string;
+  normalize?: ServerErrorContext;
+  apiErrorMessage?: string;
+};
+
+export function withErrorBoundary<Args extends unknown[]>(
+  handler: (...args: Args) => Promise<Response>,
+  ctx: ErrorBoundaryContext,
+): (...args: Args) => Promise<Response> {
+  return async (...args: Args): Promise<Response> => {
+    try {
+      return await handler(...args);
+    } catch (error) {
+      return errorResponse(error, {
+        logger: ctx.logger,
+        event: ctx.event,
+        msg: ctx.msg,
+        normalize: ctx.normalize,
+        apiErrorMessage: ctx.apiErrorMessage,
+      });
+    }
+  };
+}
+
+export function asRouteErrorContext(input: {
+  request: NextRequest;
+  route: string;
+  method?: string;
+  requestId?: string;
+}): Record<string, unknown> {
+  return {
+    route: input.route,
+    method: input.method ?? input.request.method,
+    ...(input.requestId ? { requestId: input.requestId } : {}),
+  };
+}
+
+export function normalizeForResponseOnly(
+  error: unknown,
+  normalize?: ServerErrorContext,
+): NormalizedServerError {
+  return normalizeServerError(error, normalize);
+}
diff --git a/src/lib/server/logger.ts b/src/lib/server/logger.ts
index aff248d..cacb6dc 100644
--- a/src/lib/server/logger.ts
+++ b/src/lib/server/logger.ts
@@ -6,31 +6,6 @@ import pinoPretty from 'pino-pretty';
 export type ServerLogger = Logger;
 export type ServerLogLevel = 'error' | 'warn' | 'info';
 
-export type ServerErrorClass =
-  | 'validation'
-  | 'auth'
-  | 'permission'
-  | 'upstream'
-  | 'storage'
-  | 'db'
-  | 'timeout'
-  | 'unknown';
-
-export type ServerErrorContract = {
-  errorClass: ServerErrorClass;
-  retryable: boolean;
-  httpStatus: number;
-  operation: string;
-  degraded: boolean;
-  cause?: unknown;
-};
-
-export type ServerApiErrorBody = {
-  error: string;
-  retryable?: boolean;
-  details?: Record<string, unknown>;
-};
-
 export type LoggedError = {
   name: string;
   message: string;
@@ -141,16 +116,6 @@ export function errorToLog(error: unknown, depth = 0): LoggedError {
   };
 }
 
-export function apiErrorBody(
-  input: ServerApiErrorBody,
-): ServerApiErrorBody {
-  return {
-    error: input.error,
-    ...(typeof input.retryable === 'boolean' ? { retryable: input.retryable } : {}),
-    ...(input.details ? { details: input.details } : {}),
-  };
-}
-
 export function hashForLog(value: string | null | undefined): string | null {
   const normalized = typeof value === 'string' ? value.trim() : '';
   if (!normalized) return null;
diff --git a/tests/unit/server-error-contract.spec.ts b/tests/unit/server-error-contract.spec.ts
new file mode 100644
index 0000000..84307d7
--- /dev/null
+++ b/tests/unit/server-error-contract.spec.ts
@@ -0,0 +1,55 @@
+import { expect, test } from '@playwright/test';
+import {
+  ServerAppError,
+  createServerAppError,
+  isServerAppError,
+  normalizeServerError,
+  toApiErrorBody,
+  toHttpStatus,
+} from '../../src/lib/server/errors/contract';
+
+test.describe('server error contract', () => {
+  test('normalizes unknown throwable to fallback shape', () => {
+    const normalized = normalizeServerError('boom');
+    expect(normalized.code).toBe('UNKNOWN_SERVER_ERROR');
+    expect(normalized.errorClass).toBe('unknown');
+    expect(normalized.httpStatus).toBe(500);
+    expect(normalized.retryable).toBe(false);
+    expect(normalized.message).toBe('boom');
+  });
+
+  test('preserves ServerAppError metadata', () => {
+    const err = createServerAppError({
+      code: 'USER_PROGRESS_UPDATE_FAILED',
+      message: 'Failed to update progress',
+      errorClass: 'db',
+      retryable: true,
+      httpStatus: 500,
+      details: { operation: 'update_progress' },
+    });
+    const normalized = normalizeServerError(err);
+    expect(isServerAppError(err)).toBe(true);
+    expect(normalized.code).toBe('USER_PROGRESS_UPDATE_FAILED');
+    expect(normalized.errorClass).toBe('db');
+    expect(normalized.httpStatus).toBe(500);
+    expect(normalized.retryable).toBe(true);
+    expect(normalized.details?.operation).toBe('update_progress');
+  });
+
+  test('maps normalized errors to API body + status', () => {
+    const normalized = normalizeServerError(
+      new ServerAppError({
+        code: 'UPSTREAM_TTS_ERROR',
+        message: 'Upstream failure',
+        errorClass: 'upstream',
+      }),
+    );
+    const body = toApiErrorBody(normalized, { includeDetails: false });
+    expect(body).toEqual({
+      error: 'Upstream failure',
+      errorCode: 'UPSTREAM_TTS_ERROR',
+      retryable: true,
+    });
+    expect(toHttpStatus(normalized)).toBe(502);
+  });
+});
diff --git a/tests/unit/server-error-response.spec.ts b/tests/unit/server-error-response.spec.ts
new file mode 100644
index 0000000..a5ed779
--- /dev/null
+++ b/tests/unit/server-error-response.spec.ts
@@ -0,0 +1,43 @@
+import { expect, test } from '@playwright/test';
+import { errorResponse } from '../../src/lib/server/errors/next-response';
+import { createServerAppError } from '../../src/lib/server/errors/contract';
+
+test.describe('server error response helper', () => {
+  test('returns mapped 4xx response with explicit app code', async () => {
+    const response = errorResponse(
+      createServerAppError({
+        code: 'AUTH_UNAUTHORIZED',
+        message: 'Unauthorized',
+        errorClass: 'auth',
+        httpStatus: 401,
+        retryable: false,
+      }),
+      {
+        apiErrorMessage: 'Unauthorized',
+      },
+    );
+
+    expect(response.status).toBe(401);
+    await expect(response.json()).resolves.toEqual({
+      error: 'Unauthorized',
+      errorCode: 'AUTH_UNAUTHORIZED',
+      retryable: false,
+    });
+  });
+
+  test('normalizes unknown errors to safe 500 without stack leakage', async () => {
+    const response = errorResponse(new Error('sensitive internal details'), {
+      apiErrorMessage: 'Internal Server Error',
+      normalize: { code: 'UNKNOWN_SERVER_ERROR', errorClass: 'unknown' },
+    });
+    const body = await response.json();
+    expect(response.status).toBe(500);
+    expect(body).toEqual({
+      error: 'Internal Server Error',
+      errorCode: 'UNKNOWN_SERVER_ERROR',
+      retryable: false,
+    });
+    expect(JSON.stringify(body)).not.toContain('stack');
+    expect(JSON.stringify(body)).not.toContain('cause');
+  });
+});